1. Field of the Invention
The present invention relates to a conductive high molecular composition useful for the manufacture of electronic circuits. More particularly, the present invention relates to a conductive high molecular composition comprised of a high molecular weight substance incorporated with a specific metal salt, which has an insulating property in the normal state but functions as an electroconductive material only in the event it is interposed between slightly spaced metal films and heated.
2. Description of the Prior Art
What is carried out from the past for the formation of electronic circuits is to bond electronic parts to circuit substrates or to laminate such circuit substrates to form multilayer integrated circuits. Generally adopted for bonding one circuit substrate to the other or electronic parts to such circuit substrates is a method wherein the surfaces of the metal films on the individual circuit substrates or electronic parts are bonded with a solder. A pasty solder is usually employed for soldering a number of minute spots in an efficient and precise manner.
A conventional pasty solder is in the form of a pasty dispersion of a powder of a soldering alloy such as a tin-lead alloy and a flux in a high boiling point solvent such as a mineral spirit functioning as a viscosity-regulating vehicle. The use of such pasty solder for forming an electronic circuit on a substrate metal of a circuit panel generally necessitates the steps of applying the pasty solder in a given configuration onto the surface of a panel having electronic parts thereon by way of screen printing or by the aid of a dispenser and heating the panel to melt the powder of the soldering alloy dispersed in the pasty solder and attaching it onto the surface of the substrate metal by a cooperative action of the flux. When the pasty solder is heated, the powder of the soldering alloy is molten and any oxide existing on the surface thereof is reduced by the action of the flux to form pure molten soldering alloy particles having a clean surface. At the same time, the substrate metal on the panel is also reduced to have a clean surface. Thus, the molten soldering alloy particles are improved in their coagulating force and wettability for the substrate metal, and as a result, a film of the soldering alloy is formed on the surface of the substrate metal.
In such a pasty solder, the soldering alloy exists in the form of discrete free alloy particles separated from the flux and the solvent. The size of the particles may be very small but is still too large to form a homogeneous composition together with the flux and the solvent. Many attempts are still being made to minimize the size of the alloy particles but the size is substantially limited at present to about 10 .mu. in diameter. The alloy particles should have a diameter less than 1 .mu. to form a stable homogeneous pasty solder.
In recent years, the density of the printed circuits for electronic equipment became higher so that the circuit patterns on substrates are constructed by a number of extremely thin lines. In some cases, printed circuits are constructed by a circuit pattern having lines of several hundred microns in width at similar intervals. As the density of circuit patterns becomes higher, therefore, a pasty solder therefor should be supplied in a more precise manner. In case a pasty solder is supplied by way of screen printing for the manufacture of such high density printed circuits, a screen having a smaller mesh should be used. In case a dispenser is used for supplying a pasty solder, the use of an extrusion nozzle having a smaller diameter becomes necessary as well. Since solid particles of the soldering alloy are contained in the pasty solder as described above, however, the size of the particles cannot eventually be disregarded at the time of supplying the pasty solder to substrates by way of screen printing or by the aid of a dispenser. In case of supplying a pasty solder by way of screen printing, solid particles of the soldering alloy contained therein often cause clogging of the screen so that it becomes difficult to supply the alloy particles homogeneously in a given configuration. Further, friction between the alloy particles and the screen may cause damage of a mask, thus making the life of the screen short. In case of using a dispenser for the supply of a pasty solder, the alloy particles clog the extrusion nozzle of the dispenser whereby the clogging particles function as a filter for the pasty solder passing through the nozzle to permit passage of only the flux and the solvent while leaving the solid particles of the soldering alloy, thus making it extremely difficult to extrude the pasty solder wherein the alloy particles are homogeneously dispersed.
Soldering between the alloy particles and the substrate metal is attained, as described previously, by mutual coagulation force and enhanced substrate metal-wetting property of the molten alloy particles. Since surface tension of the individual molten alloy particles is strong, however, it is difficult to allow all of the alloy particles to participate in soldering. The alloy particles not participating in soldering remain on the substrate panel and cause occurrence of unexpected short circuits between the lines of the circuit pattern. Thus, a short circuit may easily be formed by existence of even a single alloy particle between the lines constituting the circuits, thus resulting in a detrimental defect. To avoid such result, it is recommended to wash the printed circuits after soldering with an organic solvent for eliminating any residual alloy particle. Even by such washing, it is difficult to eliminate residual alloy particles entirely from the treated circuits.
Such a drawback may be overcome more or less by reducing the diameter of the particles of the soldering alloy used. However, the reduction in the diameter of the particles makes the production cost higher and the particles in the resultant pasty solder so stable that coagulation of the alloy particles may hardly occur and makes the soldering operation itself difficult.
Besides the methods above mentioned for using a pasty solder to form a metal film on the desired areas of circuit substrates, the use of an electroconductive polymeric substance comprised of a metal powder dispersed in a polymeric substance as a matrix is also known in this art for imparting electroconductivity to electric parts. From the past, there is widely known a conductive high molecular composition which is comprised of a polymeric substance such as rubber or a synthetic resin having been incorporated with a powdery conductive material such as a metal or alloy powder or carbon to provide the polymeric substance with conductivity. However, a conductive high molecular composition of such type can not be used for treating circuit substrates therewith, since such composition itself is electroconductive and will make all the metal parts on circuit substrates electroconductive when interposed between the circuit substrates.
The above mentioned various drawbacks and disadvantages of the conventional pasty solder and conductive high molecular compositions result apparently from their metal-liquid or metal-solid two heterogenous phase compositions wherein metal particles are dispersed in an organic liquid or solid medium and also from their electroconductivity in the normal state because of free metal particles contained in the compositions.
In case of using a substance for attaching electronic parts to circuit substrates or bonding such circuit substrates mutually, the substance to be used for such purpose should be of insulating property but should impart conductivity only to the desired spots of the circuit substrates. In the field of the electronic industry, therefore, there is room for making remarkable improvements in the conventional pasty solder and conductive high molecular compositions, especially in their chemical composition, in the case of using them for the above purpose. Thus, there is a strong demand for developing a new type conductive composition for treating circuit substrates without the above mentioned drawbacks.